The present disclosure relates to a semiconductor structure and a method of forming the same. More particularly, the present disclosure relates to a dual gate extremely thin semiconductor-on-insulator (ETSOI) transistor which can be used as a bio/chem sensor and a method of forming the same.
Ion sensitive field effect transistors (ISFETs) with chemically sensitive gates or chemically sensitive electrodes connected to the gate are used in a variety of applications for detecting chemical and biological species. The principal of an ISFET operation is based on the change in the accumulated charge on or near the gate/electrode that results in a change in the output current of the transistor.
There have been various schemes proposed for detecting the change in the conductance of the transistor. This includes, for example, performing the circuit readout by detecting the conductance change in a sub-threshold regime, or detecting the threshold voltage shift/current change under a constant gate bias at linear or saturation regimes.
However, additional circuitry will be required to properly detect the small change in the output signal due to the change in the total charge as a result of the chemical reaction which is small in nature. This will as a result govern the size of the sensor, i.e., the dimension of the chemical sensitive gate of the chemical sensitive electrode, to allow a sufficiently large number of reactions to occur in order to result in a sufficiently large output signal.
The requirement for scaling up the sensor dimension to facilitate the detection will in turn limit the size of an array for applications which rely on parallel detection such as DNA sequencing. Additional challenges associated with the operation of an ISFET include the drift effects due to, for example, temperature and charge trapping, which thus require additional circuitry to compensate the sensor drift. There are also potential complexities in particular with the fabrication process of the microwells that are required for array sensors to isolate each sensor from its neighboring device including bonding and alignment accuracy, and potential compromise in sensor detection limit due to the increase in the gate capacitance of the ISFET as a result of the back-end of the line (BEOL) parasitic capacitances and additional oxide coatings in the wells.
In view of the above, there is a need for providing a new ISFET which overcomes the drawbacks associated with prior art ISFET devices.